It is well known that in North America, lime and cement is typically produced in rotary kilns. The primary energy consuming reaction in both processes is the conversion or calcination of limestone (CaCO3) to lime (CaO) also known as quicklime. The intense heat and energy needed to drive the reaction requires the use of fossil fuels. A byproduct of these processes is fine particulates trapped in air pollution control systems of rotary kilns commonly known as lime kiln dust (LKD) and cement kiln dust (CKD). LKD and CKD contain, primarily, limestone, fly ash from the fuel and quicklime.
The required use of fossil fuels can present pollution concerns and, depending upon the type of fuel used, may require a further unit operation, such as flue gas desulfurization, to satiate the problem. In furtherance, typical exit temperatures of kilns exceed that which can be effectively used for dry scrubbers. The dry scrubbing operation is best suited for temperatures in the range of 350° F. to 400° F. The discharge or offgas temperature of a long, straight-type kiln, for example can be in excess of 1000° F.
To achieve effective dry scrubber operation, the excess heat must be extracted from the offgas. Typically, it is dumped to the surrounding environment and wasted.
This is less energy-efficient, represents a sub-optimum economic condition, and contributes to long-term environmental concerns such as the production of green house gases.
Conventional boilers are commonly used to capture heat contained in offgases from fossil fuel-fired sources. Operational parameters and materials of construction must be carefully controlled in order to avoid corrosive chemical attack. Typically of greatest concern is in-situ formation of sulfurous acidic compounds. When burning fossil fuels containing sulfur, corrosive acidic formations will occur on boiler surfaces that are below acid dew temperature.
A gas scrubbing method is established in United States Patent Publication No. US2005/0031515, published Feb. 10, 2005, and incorporated herein by reference.